1. Field of the Invention:,
The present invention relates to an erase device for a magnetic layer memory having a storage medium being moved relatively with respect to a combined magnetic head system including a magnetic erase head having a one-part coil winding that is connected to a DC voltage source via a switch device in order to produce an alternating current of predetermined erase frequency in the winding of the erase head dependent on a high-frequency control pulse sequence being supplied to the switch device.
2. Description of the Prior Art
A multitude of applications is known wherein audio, image or data information is stored in erasable fashion on a magnetic layer storage device that is fashioned as a magnetic tape or as a magnetic disk. For recording or, respectively, reading information, the storage medium is moved relative to a magnetic write/read head. During the recording event, the coil current is modulated in dependence upon information to be recorded, flows through the magnetic head and effects on the storage medium a remanent magnetization changing corresponding to the modulation of the current. During the read event, conversely, a voltage signal of a waveform corresponding to the stored information is induced in the magnetic head by the remanent magnetization on the storage medium. Magnetic layer memories of this type are generally utilized as re-recordable memories, i.e. it must be possible to erase stored information either in predetermined regions of the storage medium or on the entire stored medium. In addition to the read/write head, a separate erase head being individual controlled is provided for that purpose.
Given the multitude of applications of magnetic layer memories of this type, a plurality of devices for erasing stored information is also known. The most simple solution for erasing stored information is in supplying a direct current to the erase head and therefore generating a DC magnetic field that effects a uniform magnetization of all magnetic particles with a given polarity on the passing storage medium. Apart from special instances, this erase procedure has had no essential significance in practice for reasons to be set forth below. The following consideration, therefore, refers to all of those situations wherein an AC magnetic field is generated with the erase head, the AC magnetic field produces, when viewed statistically, a randomly-distributed magnetic orientation of the individual magnetic particles on the passing storage medium. In consequence, no privileged magnetic direction can be detected on the erased region of the storage medium, even when viewed locally.
The oldest embodiment of a magnetic erase head is known from audio tape recorders. This known erase head is constructed of two core halves that each carry respectively one of two subwindings connected in series with one another. Simply viewed, this can be considered an erase head having a single magnetic coil whose resonant frequency is tuned to the erase frequency.
This erase head structure having a single magnetic coil grounded at one side is conventionally utilized only in magnetic tape recorder devices for audio recording. This is to be attributed to the fact that the erase frequencies in this application are orders of magnitude lower than in other applications, for example in magnetic data memories. In magnetic data memories, the erase frequencies lie in a range of 5 MHz and above, whereas erase frequencies up to 75 kHz are entirely adequate for applications in the audible range, even given high-fidelity quality. In erase heads that have resonant frequencies tuned to such low erase frequencies, high O coil qualities O can be achieved. Consequently, a low erase current is sufficient. This, in turn, means that the drive circuit for the erase head can be adapted to low currents and be constructed simply and cost-effectively because filters and low-distortion amplifiers can be designed correspondingly.
These advantages no longer apply to erase heads having high erase frequencies as must be employed, for example, in magnetic data memories. Moreover, in such applications, a further property of the erase head having a single magnetic coil forms a serious constraint, this to be attributed to the single-sided grounding thereof. This ground connection of the return branch gives rise for a relatively long loop that is the cause of electromagnetic noise emission given erase heads having a high erase frequency. This electromagnetic noise emission is difficult to deal with even given extremely carefully manufactured connections to ground.
In magnetic data memories, therefore, the erase heads are usually provided with a center-tapped magnetic coil. Switch elements are connected to the coil ends, whereas the center tap is supplied with a DC voltage. The switch elements, preferably switching transistors, are driven by complementary switching signals that therefore alternately apply one of the coil ends to ground or to a potential that is usually lower than that of the DC voltage supplied to the center tap. The alternating on time of the two switch elements respectively amounts to 50% of a switching period in the ideal case. The direction of the magnetic field generated by the magnetic coil is therefore reversed at the end of each half cycle of the switching period. An alternating magnetic field is produced in this manner at the gap of the erase head. An example of this type of a magnetic erase head together with its drive circuit is disclosed in U.S. Pat. No. 4,466,027, fully incorporated herein by this reference.
One problem in this known design of magnetic erase heads is that the desired symmetry of the two sub-windings of the magnetic coil can only be realized incompletely. The effects of this asymmetry that occurs in practice can be reduced by increasing the number of turns. However, the number of turns must be reduced in high-frequency erase heads having a center-tapped magnetic coils, so that winding errors have all the greater influence. In addition to the inherently-high erase frequency, it must also be taken into consideration that the natural resonance of the magnetic coil usually is set at least somewhat higher than the erase frequency in order to have balancing latitude for the compensation of manufacturing tolerances in view of this resonant frequency. Asymmetry is also to be attributed to the fact that the magnetic coil usually is wound bifilarly for reasons of costs. The higher stray capacitance that is unavoidable given a bifilar winding additionally takes effect here.
Due to the significance of the symmetry for the erase event, it is expedient to explain this problem in yet somewhat greater detail. As already results from the hysteresis curve, each magnetic storage medium is a medium having pronounced nonlinear properties. It is the purpose of each erase arrangement to generate a strong magnetic field that fully penetrate into the magnetic storage medium with a magnetic field strength that optimally largely corresponds to the saturation magnetization, so that all magnetic particles, i.e. local magnets, become magnetically saturated. Since the storage medium is moved relative to the erase head, the alternating frequency of the erasing magnetic field must be selected additionally high in comparison to this relative speed. Only then can it be assured that the magnetization of the individual local magnets, as seen on statistical average, does not have any privileged direction. This, however, can only be achieved when the on times of the two switch elements are optimally identical and, moreover, when the field strengths of the two components of the alternating magnetic field also coincide optimally accurately in terms of magnitude. Already if just one of these two conditions is not adequately met, the desired statistical distribution of the magnetization direction of the local magnets during the erase event or, from a more general point of view an undirected magnetization is not achieved and, instead, a DC field component is recorded.
The same situation can also be evaluated in the following manner with respect to a defined local area on the storage medium The effect of the alternating magnetic field in this local area can be interpreted by a field strength vector which becomes inversely proportional to the time span of its influence on the storage medium. In order to meet the desired conditions, the spectrum of this radiation must consist of sinusoidal harmonics such that all of the 2n.sup.th harmonics are zero, where D is a positive integer.
The reason for such strict requirements is that, as known and is likewise explained in the aforementioned U.S. Pat. No. 4,466,027, each remaining DC field component causes distortions in the renewed recording of new information. When recording analog information, signal distortions resulting from the superimposition of the remanent DC field component with the analog signal to be recorded; in digital recording, the effect thereof is known as peak shift.
This, however, is only one of the influencing variables for the peak shift that occurs in practice. Expressed in general terms, the peak shift is dependent on the recorded wavelength, i.e. is also influenced by the recording method employed and, as known, is also influenced by the band width of the read channel. For example, the peak shift in a read channel having a narrow band-pass characteristic is greatest for long-wave signal portions. Finally, the degree of the peak shift is also influenced by the amplitude of the pre-magnetization current when a pre-magnetization is utilized in the recording event in order to linearize the same. The coercivity of the storage medium and the temperature response during operation of the storage medium also influences the degree of peak shift. As a consequence of the variety of these influencing variables, peak shift values can be very quickly reached in the worst case which cannot be accepted by the storage device. It is therefore important to minimize the influence of each variable likely to generate distortions.
In the present context, the influence of DC field components on the recording quality is known, for example, from "Magnetic Recording", Vol. II: "Computer Data Storage", edited by C. Denis Mee and Eric D. Daniels, McGraw Hill, 1988, page 206. Systemically in the NRZI recording method, for example, a DC component that cannot be left out of consideration is constantly co-recorded as a part of the spectrum of the signals to be recorded, i.e. a DC component or low-frequency components that have amplitudes too significant to be left out of consideration are already inherently contained in the recording signal just because of the use of this recording method. It can be immediately seen that any DC field magnetization remaining on the magnetic storage medium after the erase event then acts as a further distortion in the signal recording because it superimposes on the signal components.
Finally, yet another aspect should be pointed out, which is important for the circuit design of, for example, magnetic tape recorder cassette devices for data storage but also of battery-operated audio tape recorder devices. In the former periphery storage devices, only supply voltages of +5 volts and +12 volts are usually available. The same applies to battery-operated audio or, respectively, video recorder devices. This means a design limitation in view of the components selection. For reason of cost, finally, attempts are to be made to optimize the coil impedance to a high value insofar as possible in an erase head having a high erase frequency and to keep the required coil current optimally low. Only by keeping the coil current low is it possible to utilize less expensive switching transistors in the drive circuit. This is particularly true when the drive circuit is to be constructed as an integrated circuit, since the permissible current load of integrated circuits is frequently even lower than circuits composed of discrete components.